Long term availability of metals in sludge amended acid soils

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Abstract
Nicotiana tabacum L. was grown to maturity in 1984 at two research sites in Maryland which were amended with municipal sludge materials representing a variety of wastewater treatment technologies in 1972, 1976 and 1978. Four sludges from Washington, D.C. and Annapolis, MD were applied at rates equal to 0, 56, 112 and 224 Mg ha‐1. A high‐metal sludge from Chicago was applied at rates equal to 50 and 100 Mg ha‐1 in 1978. Composite leaf samples of cured tobacco and Ap horizon soil samples were analyzed for Zn, Cu, Mn, Fe, Pb, Ni and Cd. Available soil metals were estimated using three procedures: DTPA (pH 7.3), Double Acid and Mehlich 3. DTPA and Double Acid produced significant correlation coefficients (r) for Zn, Cu, Mn, Ni and Cd when compared with plant metal contents; however, none of the extractants produced significant r values for Fe or Pb. All sludge sources resulted in significant increases in DTPA soil Zn, Cu, Ni and Cd with increased application rates; however, plant levels were affected most when soil pH values were below 5.8. Plant and soil levels typically exhibited quadratic responses with increased metal loading. The application of limed sludge greatly reduced plant metal contents.

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